Powder coatings, which are dry, finely divided, free flowing, solid materials at room temperature, have gained considerable popularity in recent years over liquid coatings for a number of reasons. For one, powder coatings are user and environmentally friendly materials, since they are virtually free of harmful fugitive organic solvent carriers that are normally present in liquid coatings. Powder coatings, therefore, give off little, if any, volatile materials to the environment when cured. This eliminates the solvent emission problems associated with liquid coatings, such as air pollution and dangers to the health of workers employed in coating operations.
Powder coatings are also clean and convenient to use. They are applied in a clean manner over the substrate, since they are in dry, solid form. The powders are easily swept up in the event of a spill and do not require special cleaning and spill containment supplies, as do liquid coatings. Working hygiene is, thus, improved. No messy liquids are used that adhere to worker's clothes and to the coating equipment, which leads to increased machine downtime and clean up costs.
Powder coatings are essentially 100% recyclable. Over sprayed powders can be fully reclaimed and recombined with the powder feed. This provides very high coating efficiencies and also substantially reduces the amount of waste generated. Recycling of liquid coatings during application is not done, which leads to increased waste and hazardous waste disposal costs.
In the past, most powder coating was performed on metals which can withstand high temperatures at which many conventional coating powders fuse and cure. Recently, however, several coating powders have been developed for substrates, such as wood, fiberboard, certain plastics, etc., which require coating powders which fuse (in the case of thermoplastic coating powders) or fuse and cure (in the case of curable coating powders) at relatively low temperatures. Examples of such coating powders are found, for example, in U.S. Pat. Nos. 5,824,373, 5,714,206, 5,721,052, and 5,731,043, the teachings of each of which are incorporated herein by reference. Low temperature coating prevents charring of the substrate and helps to prevent excessive outgassing of moisture.
One coating powder chemistry is based on glycidyl (meth)acrylate copolymers as the resin and a multi-functional carboxylic acid, e.g., monomer or polyester, as the cross-linking agent. Such coating powders are taught in U.S. Pat. No. 5,436,311, the teachings of which are incorporated herein by reference. The coating powders taught in U.S. Pat. No. 5,436,311 are primarily suitable for coating substrates, such as metal, which are not heat sensitive. Such coatings tend to exhibit good weatherability, and it would be desirable to utilize such coating powder chemistry for heat sensitive substrates. A general problem with producing coating powders for heat-sensitive substrates is the narrow temperature differential between extrusion temperatures and curing temperatures. The components of the coating powder composition must be heat-fusible in an extruder without substantial curing taking place. At the same time, they must fuse and cure at a temperature not a great deal higher so as to be useful for coating heat sensitive substrates. For example, extrusion temperatures may reach 200.degree. F. (93.degree. C.) while curing may be desired at 275.degree. F. (135.degree. C.).
Catalysts, such as phosphonium bromide, are utilized in low temperature-curing coating powders to speed up the cure and thereby permit curing at lower time/temperature parameters. However, catalysts also tend to promote unwanted pre-curing during the extrusion process. Such pre-curing is often expressed as increased gloss in the cured coating. For many applications, reduced gloss in a coating is a desired characteristic. Matting agents may be used, for example, to reduce gloss. Pre-curing of the coating powder during extrusion process counteracts measures to reduce gloss, such as matting agents.
Another epoxy which cross-links with carboxylic acid functional polyesters and which is commonly used in coating powders is triglycidyl isocyanurate (TGIC). An example of such a coating powder is found in U.S. Pat. No. 5,880,223, the teachings of which are incorporated herein by reference. The powder coatings in this patent are primarily suitable for conventional substrates, such as metals, although it is taught that with selection of an appropriate catalyst the composition is suitable for coating on wood. An example of such a catalyst is ethyltriphenylphosphonium bromide. It is found that phosphonium bromide salts tend to destabilize coating powders based on epoxy/carboxylic acid functional polyester chemistry particularly when the combined epoxy functionality and the combined carboxylic acid functionality of the polyester is high.
Other coating powders are based on epoxy resin and carboxylic acid functional acrylic resins. Examples of such powders are found in U.S. Pat. No. 5,744,522, the teachings of which are incorporated herein by reference.